A new mixing length adapted to the constraints of the hectometric-scale gray zone of turbulence for neutral and convective boundary layers is proposed. It combines a mixing length for mesoscale ...simulations, where the turbulence is fully subgrid and a mixing length for Large-Eddy Simulations, where the coarsest turbulent eddies are explicitly resolved. The mixing length is built for isotropic turbulence schemes, as well as schemes using the horizontal homogeneity assumption. This mixing length is tested over three boundary layer cases: a free convective case, a neutral case and a cold air outbreak case. The later combines turbulence from thermal and dynamical origins as well as presence of clouds. With this new mixing length, the turbulence scheme produces the right proportion between subgrid and resolved turbulent exchanges in Large Eddy Simulations, in the gray zone and at the mesoscale. This opens the way of using a single mixing length whatever the grid mesh of the atmospheric model, the evolution stage or the depth of the boundary layer.
In urban canopies, the variability of pollution may be influenced by the presence of surface heterogeneities like orography and buildings. Using the Meso-NH model enhanced with an immersed boundary ...method (IBM) to represent accurately the impact of the 3D shape of buildings on the flow, large-eddy simulations are performed over city of Toulouse (France) with the dispersion of a plume following a plant explosion on 21 September 2001. The event is characterized by a large quantity of nitrogen dioxide released in a vertical column after the explosion, quickly dispersed by a moderate wind prevailing in the lower atmospheric layers. Assuming a passive pollutant, the model develops a realistic plume dispersion. A sensitivity analysis of the advection scheme to the spread is presented. The limited population’s exposure to pollution developed by the model appears in good agreement with previous health studies. Beyond this case, IBM is a promising way to represent flow interaction with buildings and orography in atmospheric models for urban applications.
Coupled atmosphere‐fire modeling is recognized as a relevant approach for the representation of the interaction between a wildland fire and local meteorology at landscape scales. The atmospheric ...model component used in the coupled system is based on several approximations, which are adopted for computational efficiency or physical processes representation, including the widely used anelastic approximation. The validity domain of the anelastic approximation may be questioned in the context of high‐resolution wildland fire modeling due to the large fire‐induced heat releases near the surface. This study aims to study this question with the MesoNH anelastic model coupled with the Blaze fire model. A compressible version of the MesoNH‐Blaze coupled model has been developed for comparison with the anelastic system. The FireFlux I experimental fire is used for this comparative study conducted at a 10‐m and a 25‐m horizontal atmospheric resolution. Results show significant anelastic/compressible differences at a 10‐m resolution on the physical processes occurring near the fire with higher horizontal velocities and the presence of gravity waves downstream of the fire. This is in addition to the fire plume with realistic larger vertical velocities. Differences at a 25‐m resolution are much smaller in all evaluated processes. The compressible system only enriches the physics underlying fire‐atmosphere interactions at a very high resolution, which means that the anelastic approximation remains relevant for large‐scale coupled atmosphere‐fire simulations, considering the significant economy concerning numerical costs.
Plain Language Summary
Wildfires burn large amounts of forests each year, destroying the living habitat of many species, endangering human settlements and provoking cross‐continent smoke events leading to air quality issues. Wildfires result from complex physical, chemical and biological processes. Understanding the fundamental processes driving wildfire behavior is a key point for the prediction of fire spread across the landscape and the induced atmospheric dynamics. Numerical models coupling the atmospheric dynamics (wind, temperature, air density and pressure, etc.) and fire front evolution are efficient tools in this scientific process. This paper evaluates how such model simulations are affected by the way in which the effects of high air density gradients induced by the very large amount of heat released into the atmosphere are modeled. When the spatial resolution of the atmospheric model is as high as 10 m, an exact – compressible – formulation leads to the formation of gravity waves downstream of the fire, and to stronger fire‐induced wind, fire propagation and larger vertical velocities than an approximate (and computationally faster) approach. At a resolution of 25 m, which is still very high, the approximate approach leads to results, which are as good as the exact one. This can then be used to simulate wildland fire behavior at landscape‐to‐meteorological scales. It also paves the way for future accurate wildfire forecast systems.
Key Points
A compressible version of the MesoNH atmospheric model has been developed to evaluate equation system approximations in wildfire simulation
Horizontally buoyancy‐driven effects induce turbulent movements, waves ahead of the fire, and a bending of the plume
The anelastic approximation for the atmosphere model is suitable to represent wildland fire effects, except for resolutions of 10 m or finer
The Aullene fire devastated more than 3000 ha of Mediterranean maquis and pine forest in July 2009. The simulation of combustion processes, as well as atmospheric dynamics represents a challenge for ...such scenarios because of the various involved scales, from the scale of the individual flames to the larger regional scale. A coupled approach between the Meso-NH (Meso-scale Non-Hydrostatic) atmospheric model running in LES (Large Eddy Simulation) mode and the ForeFire fire spread model is proposed for predicting fine- to large-scale effects of this extreme wildfire, showing that such simulation is possible in a reasonable time using current supercomputers. The coupling involves the surface wind to drive the fire, while heat from combustion and water vapor fluxes are injected into the atmosphere at each atmospheric time step. To be representative of the phenomenon, a sub-meter resolution was used for the simulation of the fire front, while atmospheric simulations were performed with nested grids from 2400-m to 50-m resolution. Simulations were run with or without feedback from the fire to the atmospheric model, or without coupling from the atmosphere to the fire. In the two-way mode, the burnt area was reproduced with a good degree of realism at the local scale, where an acceleration in the valley wind and over sloping terrain pushed the fire line to locations in accordance with fire passing point observations. At the regional scale, the simulated fire plume compares well with the satellite image. The study explores the strong fire-atmosphere interactions leading to intense convective updrafts extending above the boundary layer, significant downdrafts behind the fire line in the upper plume, and horizontal wind speeds feeding strong inflow into the base of the convective updrafts. The fire-induced dynamics is induced by strong near-surface sensible heat fluxes reaching maximum values of 240kWm−2. The dynamical production of turbulent kinetic energy in the plume fire is larger in magnitude than the buoyancy contribution, partly due to the sheared initial environment, which promotes larger shear generation and to the shear induced by the updraft itself. The turbulence associated with the fire front is characterized by a quasi-isotropic behavior. The most active part of the Aullene fire lasted 10 h, while 9 h of computation time were required for the 24 million grid points on 900 computer cores.
Snow accumulation in alpine terrain is controlled by three main processes that act at different spatial scales: (i) orographic snowfall, (ii) preferential deposition of snowfall, and (iii) ...wind‐induced snow transport of deposited snow. The relative importance of these processes largely remains uncertain at small scale (10–100 m). This study presents how high‐resolution coupled snowpack/atmosphere simulations help quantifying the effects of these processes. The simulation system consists of the detailed snowpack model Crocus and the atmospheric model Meso‐NH used in Large Eddy Simulation mode. Dedicated routines allow the coupled system to explicitly simulate wind‐induced snow transport. Our case study is a snowfall event that occurred in February 2011 in the French Alps. Three nested domains at 450, 150 and 50 m grid spacing allow the model to simulate the complex 3D precipitation and wind fields down to fine scale. We firstly assess the ability of the coupled model to reproduce meteorological conditions during the event (wind speed and direction, snowfall amount, and blowing snow fluxes). The spatial variability of snowfall and snow accumulation is then considered. At 50 m grid spacing, snowfall presents local maxima associated with the formation of rimed snow aggregates and graupel in regions of sustained updrafts. Variograms show that the resultant spatial variability of snowfall is lower than the variability of snow accumulation when considering snow transport. Despite an overestimation of simulated blowing fluxes, our results suggest that wind‐induced snow transport is the main source of spatial variability of snow accumulation in our case study.
Key Points
Snow accumulation is simulated with a fully coupled snowpack/atmosphere model
Riming influences small‐scale snowfall pattern in alpine terrain
Wind‐induced snow transport is the main source of spatial variability
To quantify the turbulent transport at gray zone length scales between 1 and 10 km the Lagrangian evolution of the CONSTRAIN cold air outbreak (CAO) case was simulated with seven large eddy models. ...The case is characterized by rather large latent and sensible heat fluxes, and a rapid deepening rate of the boundary layer. In some models the entrainment velocity exceeds 4 cm/s. A significant fraction of this growth is attributed to a strong longwave radiative cooling of the inversion layer. The evolution and the timing of the breakup of the stratocumulus cloud deck differ significantly among the models. Sensitivity experiments demonstrate that a decrease in the prescribed cloud droplet number concentration, and the inclusion of ice microphysics, both act to speed up the thinning of the stratocumulus by enhancing the production of precipitation. In all models the formation of mesoscale fluctuations is clearly evident in the cloud fields but also in the horizontal wind velocity. Resolved vertical fluxes remain important for scales up to 10 km. The simulation results show that the resolved vertical velocity variance gradually diminishes with a coarsening of the horizontal mesh, but the total vertical fluxes of heat, moisture, and momentum are only weakly affected. This is a promising result as it demonstrates the potential use of a mesh size dependent turbulent length scale for convective boundary layers at gray zone model resolutions.
Local air quality is a major concern for the population regularly exposed to high levels of air pollution. Due mainly to its aircraft engine activities during taxiing and take-off, the airport is ...often submitted to heterogeneous but important concentrations of NO xx and Particulate Matter (PM). The study suggests an innovative approach to determining the air traffic impact on air quality at the scale of the airport, its runways, and its terminals, to be able to locate the persistent high-concentration spots, for example. The pollutant concentrations at 10 m resolution and 1 s time step are calculated in order to identify the most affected areas of an airport platform and their contributors. A real day of air traffic on a regional airport is simulated, using observations and aircraft trajectories data from radar streams. In order to estimate the aircraft emissions, the Air Transport Systems Evaluation Infrastructure (IESTA) is used. Regarding local air quality, IESTA relies on the non-hydrostatic meso-scale atmospheric model Meso-NH using its grid-nesting capabilities with three domains. The detailed cartography of the airport distinguishes between grassland, parking, and terminals, allowing the computation of exchanges of heat, water, and momentum between the different types of surfaces and the atmosphere as well as the interactions with the building using a drag force. The dynamic parameters like wind, temperature, turbulent kinetic energy, and pollutants concentration are computed at 10 m resolution over the 2 km × 4 km airport domain. The pollutants are considered in this preliminary study as passive tracers, without chemical reactions. This study aims at proving the feasibility of high-scale modelling over an airport with state-of-the-art physical models in order to better understand the repartition of pollutants over an airport, taking into account advection and turbulence in interactions with buildings and regional trends, emissions, Auxiliary Power Units (APU), taxiing, parking, take off. All these processes drive the model at each time step and are not averaged over one hour or more like in Gaussian or Lagrangian ones. This study is investigating the feasibility of high spatio-temporal air quality modelling for research purposes but not for operational forecasting.
The offshore wind market is expected to see massive growth worldwide in the next decade. Recent research in this sector has led to a new generation of wind turbines characterized by larger rotors and ...higher capacity factors. Furthermore, the trend is to develop larger and denser wind farms which leads to potential deeper interactions with the troposphere. In order to provide an accurate modeling of the processes between the wind turbines, the surface and the atmospheric boundary layer, a new numerical tool has been developed. The ALM has been implemented into the opensource non-hydrostatic mesoscale atmospheric model Meso-NH used in the Large-Eddy Simulation (LES) framework. First, the tool is validated by comparing simulated results with data acquired during the New MEXICO experiments. In particular, good correlation are obtained for normal and tangential loads along the blades and the axial PIV traverse of instantaneous wind velocity components. Once the tool is validated, its ability to reproduce the Horns Rev 1 photo case is evaluated, since it is an emblematic case of wind farm-atmosphere interaction. Simulation output are post-processed using a render algorithm, resulting in synthetic images of cloud radiance. These images show great similarity with the photographs taken. All the results highlight the capacity of this new tool to represent interactions between wind farms and the lower atmosphere with a high level of details.
Abstract
A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical ...production of subgrid turbulence clearly dominates over the thermal production, whereas the situation is reversed for resolved turbulence, the tipping point occurring near the 250-m scale. For cloud dynamics, the toroidal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called subsiding shell composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy, and turbulence of the inner and outer edges of the cloud, the cloud interior, and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation in which evaporative cooling is suppressed indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling also has an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud.
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